The electrostatic optical modulator employing a resilient electrode moving over a static electrode has been a subject of multiple patents and publications. The difference between these modulators originates essentially from the shape of electrodes involved as well as the direction and spatial limitations of movement of the flexible electrode relative to the static electrode, see e.g. U.S. Pat. Nos. 4,229,075; 4,208,103 and 4,786,149. Lateral membrane movement is used in optical shutter and display of U.S. Pat. No. 6,288,824, wherein the electrostatically moved membrane and static electrode consist of periodic metal stripes, so that lateral membrane movement opens or closes multiple slits to pass or shut the light. Another approach, see U.S. Pat. No. 6,600,474, relies on a flipping of the membrane by 180 degrees, and latching it at this position by an electrostatic force. No resilient force is applied, since the membrane freely turns around the hinges between two extreme positions.
Various examples of the electrostatic optical modulators are based on the field induced bending of a cantilever membrane which is moved toward a flat static electrode, thereby changing the optical state, see U.S. Pat. Nos. 3,553,364; 3,600,798; 4,229,732; 4,731,670 and 5,781,331. Another approach relies on an electro-statically induced twisting of the membrane mirror from its initial flat position to vary the optical reflection, see U.S. Pat. Nos. 3,746,911 and 4,710,732. This electrostatic optical modulator, known as Deformable Mirror Device (DMD), is currently commercially used in a projection display.
The DMD represents a matrix of densely packed mirrors reflecting light into the objective lens when the pixels are unbiased (“on state”) and moving the reflected light out of the objective lens when they are electrostatically tilted by ˜10 degrees (“off state”). The limitations of application of the DMD strictly to projection display originates from small tilt of the membrane, since 10 degree rotation is not enough to use the DMD pixels for the flat panel displays. In the proposed shutter and display, according to the present invention, this limitation is lifted due to different pixel design which allows for membrane tilt of 90 degrees, thereby making it suitable for application to the flat panel displays.
In the original design of the previous patent on this subject, shown in FIG. 1 as a prior art, the shutter contains a membrane disposed over and separated from the substrate by an air gap, see cross-sectional view of the pixel in FIG. 1a. The membrane is held parallel to the substrate by four pillars grown on the substrate. The pillar height controls the air gap space. Two left pillars support the membrane through conductive belts attached to the membrane, see FIG. 1b, where the top view of the pixel is shown, while two other pillars support the membrane at its opposite side.
In the original design, the membrane consisted of several layers:
the bottom conductive layer connected to two supporting pillars through the conductive belts;
the insulator above the bottom conductive layer; and
the top layer having high optical reflectivity in one shutter embodiment and black, light absorbing surface in another, see below.
The top layer is isolated form the bottom conductive layer on the membrane and does not participate in the process of electrical activation of the shutter. The bottom conductive layer of the membrane, made as a conductive stripe, about a few μm wide and connected to two membrane supporting hinges, occupies a small portion of the membrane and serves as one of the electrodes for electric field induced membrane movement. This layer is shifted to one of the sides of the membrane and thus positions the membrane asymmetrically relative to the membrane supporting hinges.
The second controlling electrode is also made in the shape of a narrow metal stripe and placed on the substrate underneath and parallel to the bottom conductive layer of the membrane, see FIG. 1a. When a potential difference is applied between the bottom conductive layer on the membrane and the second electrode on the substrate, the electrostatic force rotates the membrane around the membrane hinges to reduce the distance between the active electrodes. If the air gap is deep enough to accommodate the width of the narrow part of the membrane containing the membrane electrode, at sufficient voltage the membrane starts rotating and eventually reaches the position normal to the substrate plane, thereby fully opening the shutter to pass the light or drastically change the shutter reflection, see FIG. 1c. Thus, unlike the previously discussed DMD, having small rotation angle for the membrane, the proposed shutter allows for a large shutter opening and therefore can be used for fabrication of the flat panel display.
In the previous patent, a bi-stability effect, based on the existence of two different voltages needed to bring the membrane into upright position and to hold the membrane in the upright position, have been discussed and used for building appropriate driving circuitry of the display comprising a two-dimensional matrix of these shutters. Two- and three-electrode pixel structures were proposed and analyzed. In the present patent, we will not discuss these issues and focus on display improvements introduced according to the present invention.